Add this to the functions.php of the active theme.

/**
 * Load Bootstrap CSS and JS from MaxCDN - the right way
 *
 * @refer https://millionclues.com/wordpress-tips/load-css-and-js-the-right-way/
 */
function mcabl1226_load_css_and_js() {
  // Load Boostrap CSS
  wp_enqueue_style( 'bootstrap-css', 'https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css' );
  
  // Load Theme's style.css
  wp_enqueue_style( 'style', get_stylesheet_uri() );
  
  // Load Boostrap JS
  wp_enqueue_script( 'bootstrap-js', 'https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js', array('jquery'), '1.0', true);
}
add_action( 'wp_enqueue_scripts', 'mcabl1226_load_css_and_js' );

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